1. Field of the Invention
This invention relates to an image forming apparatus, such as a digital copying machine.
2. Description of the Related Art
Recently, image information has come to be handled easily in the form of digital data. One of apparatuses using such technology is a digital PPC. Unlike a conventional analog PPC, it does not direct the reflected light from the manuscript optically to form an image on a photosensitive material. After the reflected light from the manuscript has been read by a CCD sensor in the form of an electric signal, it is converted into a digital signal. The digitized manuscript undergoes various processes and then is printed out on paper with a laser printer.
Converting the manuscript image into a digital signal enables various signal processes, including the correction of the input characteristic of the CCD sensor or the output characteristic of the laser printer, image enlargement and reduction, partial erasure, and frame outside erasure,
Furthermore, the coding process of the image converted into a digital signal makes it possible to compress the amount of data and store it efficiently. The stored images can be decoded into the original images in arbitrary order in which the images are to be printed out, and any number of them can be printed out on the laser printer.
Since such rearrangement has been made mechanically on the copied print output (the image-copied sheets of paper) by the use of a sorter or a stacker, it is impossible to avoid a trend for the apparatus to get increasingly large and an increase in noise. In addition, printing on sheets of paper requires the copying operation repeatedly.
The time required to encode an image differs with the nature of the image and is not constant. If a scanner (CDD sensor) reads an image at a constant speed, the process cannot keep up with the reading speed and will lose part of the read-out data unless the coding process speed is sufficiently faster than the reading speed. To avoid this problem, a page of image memory is provided and a page of image data is temporarily entered from the scanner into the image memory. Thereafter, the coding process is performed on the image on the image memory. By doing so, the difference between the reading speed of the scanner and the speed of the coding process can be absorbed.
During the coding process, however, because the scanner cannot enter the image input into the page memory, it has to wait to enter the next image input until the coding process has ended. To eliminate the waiting, two pages of image memory are provided, which enables the scanner to enter the image input into the other empty image memory even if one image memory is in the coding process. Thus, use of two pages of image memory enables the coding process and the image input from the scanner to be performed at the same time. By doing this alternately, it is possible to carry out the coding process consecutively without causing the scanner to wait to enter the input.
Furthermore, when the coded image is decoded and the decoded data is outputted to a laser printer, the process can be performed consecutively in a similar manner. Two pages of image memory are provided. At the same time that the decoded image is written into one image memory, the already decoded image in the other image memory is read out and outputted to the laser printer. By doing this alternately, it is possible to perform the process consecutively without causing the laser printer to wait to output images.
The above-described consecutive process requires two pages of image memory. To copy a manuscript with a high picture quality, it is necessary to raise the resolution at which the manuscript is read or images are printed on the laser printer.
Naturally, as the resolution gets higher, the capacity of image memory required to achieve the resolution grows enormous. For example, when a page of A4-size manuscripts is read at a resolution of 400 dpi as monochrome data with one bit per pixel, the capacity of image memory needed is about 2 Mbytes; and when the same page is read at a resolution of 600 dpi, the image memory capacity needed is about 4.4 Mbytes. Furthermore, when the same page is read as gray scale data with 8 bits per pixel, the image memory capacity needed is as enormous as eight times the above memory capacity.
As described above, to record image data requires a tremendous capacity of memory. If two pages of memory are necessary, it will be impossible to avoid problems including an increase in costs, an increase in the number of component parts, an increase in the power consumption, and an increase in the size of apparatus.